The invention relates to a power assisted steering system for vehicles, in particular passenger cars.
More and more is being demanded of power assisted steering systems as comfort requirements increase and also as a result of the interplay with other systems for controlling the dynamics of vehicle movement. Accordingly, the respective power assistance, which is in some cases relatively large, is to be ensured even in extreme situations in which changes to the usual steering behavior are particularly critical, including in terms of the driver's reaction. The ability to continuously make use of even large volume flows which this entails requires large pump capacities which reduce the engine capacity which can be used in other ways, and increases the consumption of fuel.
In order to counteract this, and especially to improve the steering sensation, DE 37 29 156 C2 provides a valve arrangement in the connection between the pump and the servo actuator which connects the forward flow and return flow of said connection as a bypass and determine, as a function of parameters which are decisive for the necessary power assistance, such as torque applied to the steering wheel by the driver, the rotary angle of the steering wheel, travel speed, pressure acting on the servo actuator and the like, the portions of the feed volume of the pump which are fed, as utilized volume portion (useful power) to the servo actuator or are fed back directly to the return flow line as an unused volume portion (power loss).
The valve arrangement which is located in the bypass is embodied as a slide valve whose slide can be charged in a pressure-compensated fashion and superimposed for the sake of spring loading on its open position by means of an actuating magnet by means of which the slide can be adjusted as a function of the current applied and actuated by means of the control unit, in direction of its blocking position which closes off the overflow cross section.
Furthermore, a power assisted steering system of the type mentioned at the beginning is known from EP 0 189 965 B1 in which a three-way flow regulator valve is located at the junction between the steering assistance pump which is embodied there as a constant delivery pump and the servo actuator and the return flow which controls the portion of the feed flow of the pump which is fed to the servo actuator as a used portion by means of an adjustment throttle whose throttle cross section can be adjusted by means of a magnet actuator as a function of parameters such as travel speed, angle of rotation of the steering wheel and the like which are decisive for the power assistance effect. The magnitude of the volume flow which passes via the cross section of the adjustment throttle or of the pressure difference which is sat in accordance with this throttle cross section is used to determine the overflow cross section or throughflow cross section to the return flow line. This is determined by means of a regulating piston which is embodied as a pressure compensator and which is charged on the one hand by the pressure provided to the adjustment throttle on the inflow side and on the other hand by the pressure which is applied to the adjustment throttle on the outflow side, supplemented by a compensation spring as actuator element.
Due to this design, irrespective of the magnitude of the opening cross section of the adjustment throttle, which at the same time forms a measurement throttle, throttle losses result with respect to the portion of the feed volume of the pump which is fed to the servo actuator as the configuration as an adjustment throttle gives rise to certain cross-sectional restrictions. This results in efficiency losses in particular with respect to small throttle cross sections for operating the steering system with a low level of power assistance. Furthermore, due to the fact that actuation is carried out by means of the control unit and the magnet actuator, such a system results in respectively corresponding pressure differences arising at the adjustment throttle in its function as a measurement throttle before the clear cross section can be correspondingly set to the return flow line by means of the regulating piston which is embodied as a pressure compensator.